1. Field of the Invention
The present invention relates to a magnetic recording medium comprising, above a non-magnetic support, a magnetic layer comprising a ferromagnetic powder and a binder, or a non-magnetic layer comprising a non-magnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder in that order. The present invention also relates to a process for producing the magnetic recording medium.
2. Description of the Related Art
Magnetic recording technology is widely used in various fields including video, audio, and computer applications since the technology has excellent characteristics that cannot be achieved by other recording methods; for example, media can be used repeatedly, systems can be built in combination with peripheral equipment due to the ease of conversion of a signal to electronic form, and signals can be corrected easily.
In general, with the demand for higher recording density of magnetic recording media for computer use, etc., it is necessary to yet further improve electromagnetic conversion characteristics, and it is important to make the ferromagnetic powder finer, the surface of the medium ultra smooth, etc.
With regard to finer magnetic substances, a recent magnetic substance employs a ferromagnetic metal powder of 0.1 μm or less or a ferromagnetic hexagonal ferrite powder having a plate size of 40 nm or less. In the case of a multilayer structure in which a magnetic layer is provided as an upper layer above a non-magnetic lower layer provided on the surface of a support, in order to highly disperse in a binder a fine non-magnetic powder used for the non-magnetic layer or the fine magnetic substance, a dispersion technique has been proposed in which the hydrophilic polar group —SO3M (M denotes a hydrogen atom, an alkali metal, or an ammonium) is introduced into the binder, and the binder chain is adsorbed on the magnetic substance or the non-magnetic powder via the polar group so as to achieve a smooth surface.
For example, a magnetic recording medium employing as binders a polar group-containing acrylic resin and a urethane resin formed from a diol having a cyclic structure and a long alkyl chain has been proposed (ref. JP-A-2002-329313 (JP-A denotes a Japanese unexamined patent application publication)). However, such binders have the problems that, since low molecular weight components are contained therein, the storage stability of the medium cannot be guaranteed, and since the polar groups are sparsely distributed in the polymer chain, there are components containing few polar groups, and adequate dispersibility and electromagnetic conversion characteristics cannot be guaranteed.
In the conventional art, a high strength, high dispersibility polyurethane resin is used as a binder, and a vinyl chloride resin is usually used in combination therewith. A vinyl chloride resin having a polar group is used in order to adjust the viscosity of a coating solution or the physical properties of a tape.
However, when used for recent data media that are required to have high capacity, a high speed, and high reliability, it has been found that a trace amount of hydrochloric acid gas generated from the vinyl chloride resin might not only affect tape storage stability but also cause head corrosion.
In particular, MR (magnetoresistive) heads, which are used in data recording systems for computer use, employ a thin metal film, and there is the problem that the characteristics might be degraded by corrosion. Furthermore, when a tape is stored for a long period of time, hydrochloric acid gas might denature the materials of the tape; for example, a fatty acid formed by hydrolysis of an ester lubricant might migrate onto the surface of the magnetic layer and crystallize. Since the recording density is higher than that of the conventional art, even very small foreign bodies have a large effect.
For these reasons, the combined use of a polyurethane resin and a polyisocyanate curing agent, employing no vinyl chloride resin, has been examined, but since due to its structure the polyurethane can only have an isocyanate-reactive hydroxyl group introduced at the polyurethane chain terminals, the crosslinking performance is degraded and the durability is insufficient compared with a case in which a vinyl chloride resin obtained by copolymerization with a hydroxyl group-containing monomer is used.
Furthermore, a homopolymer of a sulfonic acid (salt) of a diene monomer and a copolymer of a sulfonic acid (salt) of a diene monomer and various types of monomers have been proposed (ref. JP-A-10-330545). However, there are the problems that such a copolymer does not have sufficient solubility in a solvent system used for a coating solution of a magnetic recording medium, the effect thereof on the dispersibility of a fine ferromagnetic metal powder or a non-magnetic lower layer powder used in a coated ultra thin layer magnetic recording medium suitable for recent MR heads is insufficient, and required electromagenetic conversion characteristics and surface smoothness of the medium cannot be guaranteed.